1. Field of the Invention
The present invention relates to a fish eye lens system having an extreme wide field angle reaching 180 degrees, and more particularly to a fish eye lens system having both high speed and a compact size.
2. Description of the Prior Art
In the field of photography, there have been known various types of fish eye lens systems for 35 millimeter SLR cameras, such as disclosed in U.S. Pat. Nos. 3,734,600, 4,009,943, and Japanese Patent Application Laid-Open No. 54-32319 on Mar. 9, 1979.
A fish eye lens system for use in 35 mm SLR cameras generally has a front lens group of a far greater negative refractive power than that of an ordinary inverted telephoto type wide angle lens system since a fairly large back focal distance, relative to the focal length of the whole lens system, is required for avoiding an increase in size of the lens system. This extreme power distribution is apt to cause field curvature and astigmatism in the transmitted image. In addition, sagittal flare including Rinnen fehler is also increased if a high speed such as F/2.8 is desired.
To improve the field curvature and astigmatism, it is basically necessary to avoid any negative deviation of the Petzval sum to the greatest extent possible. In the prior art, this has been generally attempted by providing, at the image side of the aperture stop, at least one doublet composed of a positive lens element of a low-refractive-index-low-dispersion glass and a negative lens element of a high-refractive-index-high-dispersion glass with a cemented intermediate surface of a suitable negative refractive power formed therebetween. This approach helps avoid significant negative deviation of the Petzval sum plus also permitting correction of chromatic aberration.
The conventional lens design has taught that an additional one or more doublet, similar to the above-mentioned doublet, should be further provided on the image side of the aperture stop, alternatively the difference in the refractive index on the opposite sides of the cemented surface of the above-mentioned doublet could be increased if a still greater increase of negative refractive power in the front lens group is desired for making the lens system even further compact. The conventional lens design has also suggested forming the negative lens elements in the front lens group out of a glass of as high refractive index as possible to also reduce the size of the lens system.
With respect to the previously mentioned sagittal flare which includes Rinnen fehler, the conventional lens design for an extremely wide angle high speed lens system has permitted this aberration to remain if the desire for compactness is a preferred design criterion. This is because of the following reasons. The sagittal flare is generally a typical and fatal aberration to a lens system of high speed, and is more particularly remarkable in a case of a Gauss type lens system or an inverted telephoto type wide angle lens system. For correcting this aberration, it may be recommended to avoid locating a refracting surface having a short negative radius of curvature at a position in which an off axial light pencil passes the surface apart from the optical axis. An example of such a design technique for this purpose would be to use a glass of high refractive index for increasing the radius of curvature as much as possible. However, in case of an extremely wide angle lens system of a compact size having a field angle greater than 72 degrees and a high speed greater than F/2.8, the degree of freedom in designing the lens system is greatly restricted and a practical way to sufficiently correct the sagittal flare is still a design goal. Thus, the sagittal flare has been generally permitted to remain to some degree, and it has been excused on the basis that the remaining sagittal flare can vanish if the aperture stop is stopped down to a size with which photography is most frequently practised.
The prior art is still seeking a compact fish eye lens with minimal sagittal flare.